High intensity discharge lamps such as metal halide lamps have become increasingly used as various light sources in recent years, and such lamps are required to have long operating life.
FIG. 1 is a circuit diagram of a conventional discharge lamp lighting device for lighting a high-pressure discharge lamp. FIG. 2 is an operation waveform chart at a time when the lighting device shown in FIG. 1 starts operating, and shows temporal changes in a drive frequency of a polarity inversion (inverter) circuit, an output voltage of a down-converter, and a resonant voltage applied to the discharge lamp. In FIG. 1 and FIG. 2, a voltage supplied from a direct current power source 1 is controlled with a down-converter 2. A polarity inversion (inverter) circuit 3 is provided at an output terminal of the down-converter 2. Moreover, there is provided a serial resonance circuit 4 including a capacitor (C2) and an inductor (L3) which are connected to an output of the polarity inversion (inverter) circuit 3.
For the voltage to be applied to the discharge lamp, a pair of switching elements Q2 and Q5 and a pair of switching elements Q3 and Q4 in the polarity inversion (inverter) circuit 3 are alternately operated in a switching manner at a high frequency for a predetermined period, the high frequency being higher than a lighting frequency at the time of steady lighting.
In the case of starting to light the discharge lamp, the above-described discharge lamp lighting device turns on and off a pair of switching circuits and a pair of switching circuits, alternately, the switching circuits in each pair located diagonally from each other, and thereby generates a high-frequency voltage in a range from several tens of kilohertz to several hundreds of kilohertz between both connection terminals of each of the pairs of switching circuits. The resonance circuit 4 performs resonance boosting by use of this high-frequency voltage thereby to generate a high resonance voltage in the capacitor (C2). Then, the discharge lamp is lit by this high resonance voltage. Upon detection of lighting of the discharge lamp by use of a detection voltage detected by a voltage detection circuit 5, a control circuit turns on and off the pairs of switching circuits, alternately, to generate a low-frequency voltage in a range of several tens of hertz to several hundreds of hertz between both of the connection terminals. Thus, lighting is maintained.
Alternatively, a discharge lamp lighting device disclosed in Patent Document 1 (JP-A 2004-95334) aims to ensure a favorable starting operation even when a starting voltage is stepped up due to product variation or an end stage of a product life of a discharge lamp. To this end, the discharge lamp lighting device performs start control by turning on and off alternately a pair of switching elements Q2 and Q5 and a pair of switching elements Q3 and Q4, located diagonally from each other, while changing a drive frequency so that the drive frequency can sweep a predetermined frequency range to pass through a resonance point of a resonance circuit.
Meanwhile, from a viewpoint of downsizing components constituting the resonance circuit 4 while obtaining substantially the same voltage amplitude as that obtained in the case of performing driving at the above-described frequency, a frequency of an odd-number multiple (2n+1, n is a natural number) of a frequency of abridge portion is sometimes employed as a lighting frequency at the time of the start control. This voltage amplitude is gradually decreased as the multiplying factor becomes higher. When the frequency of the bridge portion is tripled in particular, it is possible to obtain substantially the same voltage amplitude as that obtained in a case of performing driving at a frequency equivalent to a resonance frequency f0 which is determined by an inductor serially connected to the discharge lamp and by a capacitor connected in parallel thereto, and also it is possible to achieve downsizing of the resonance circuit 4. The use of this resonance voltage of a tertiary harmonic wave for starting the discharge lamp has also been disclosed in Patent Document 2 (Japanese Patent Translation Publication No. 2005-507554).
For example, the sweeping frequency is changed stepwise while causing the frequency of the polarity inversion (inverter) circuit 3 to gradually approach the resonance point, because most of general control methods for ballasts are digital control. Even when the frequency is changed stepwise by several percent each time, the resonance voltage is not proportional to the change rate of the frequency. Instead, the resonance voltage increased according to a quadratic function is generated. For this reason, a control circuit having high resolution and capable of performing fine frequency control has been used in order to finely set up the resonance frequency.
Meanwhile, in the case of the conventional circuit, electrodes of a discharge lamp (La) may not be evenly warmed up immediately after lighting of the discharge lamp is started by use of the resonance circuit 4, in some cases. Accordingly, a high-frequency current immediately after the lighting does not flow symmetrically on positive and negative sides, but there continues a state where the current flows asymmetrically with respect to a zero current. Such discharge lamp lighting devices have been disclosed in Patent Document 3 (Japanese Patent No. 2878350) and Patent Document 4 (Japanese Patent No. 2975032), for example. In the state of asymmetric flow of the current, a high-frequency current flows having a current peak which is nearly 1.5 to 2 times as large as a current peak in a state where the current flows symmetrically. This causes large damage on the electrodes of the lamp. Moreover, the electrodes of the lamp may be severely damaged if the lamp switches to steady lighting (low-frequency lighting) while in the above-described state. In the worst case, the electrodes may break off at the bottoms.
Further, the conventional circuit is provided with a starting mode and a preheating mode, and is switched to steady lighting (low-frequency lighting). In the preheating mode, the high-frequency current is applied for a certain time period such as a fixed time period set based on estimation in advance of a time period required for allowing the high-frequency current to flow symmetrically on the positive and negative sides, or a time period set based on detection of lighting of the discharge lamp (La).
Meanwhile, one of methods of suppressing the high-frequency asymmetric current in the preheating mode takes advantage of the fact that, while the polarity inversion (inverter) circuit 3 is operating at the high frequency due to insulation breakdown of the discharge lamp, the high-frequency current flowing in the discharge lamp is restricted by impedance of inductance of the resonance circuit 4. Here, the impedance is almost ignorable when the current at the low frequency is fed at the time of steady lighting. However, the inductance of this resonance circuit 4 acts as the impedance. Accordingly, when the high-frequency current is fed, the drive frequency of the polarity inversion (inverter) circuit 3 is changed to increase the impedance serially connected to the discharge lamp, which suppresses the peak current of the asymmetric current at the start-up.
For example, assume that the inductance of the resonance circuit 4 is 100 μH, the polarity inversion (inverter) circuit 3 is operated at a high-frequency operation of 40 kHz, and the peak current (Io-p) of the asymmetric current is about 8 A (the peak current (Io-P) is about 4 A when the current is symmetric). In this case, the impedance ω of the inductance of the resonance circuit 4 is about 25Ω. To reduce the peak value of this asymmetric current approximately by half, the drive frequency of the polarity inversion (inverter) circuit 3 is raised to 80 kHz. As a result, the impedance of the inductance of the resonance circuit 4 becomes equal to about 50Ω, and the peak value of the asymmetric current is reduced by half.
In contrast, after the high-frequency current turns into a symmetric state, the high-frequency current is increased to promote preheating of the electrodes of the discharge lamp. Thus, by lowering the drive frequency of the polarity inversion (inverter) circuit 3, the impedance of the inductance of the resonance circuit 4 is reduced and the current is increased.
As described above, the drive frequency of the polarity inversion (inverter) circuit 3 is controlled to switch between a frequency for allowing the resonance circuit 4 to generate the resonance voltage at the start-up and a frequency for preheating the electrodes of the discharge lamp in the preheating mode. Once the discharge lamp is extinguished, the control has to be switched again from the preheating mode to the starting mode to change the drive frequency to such a drive frequency as to generate and supply the high voltage to the discharge lamp. Therefore, a time lag occurs for switching the control.
Moreover, since most of the general control methods for ballasts are digital control, the voltage to be changed is changed stepwise as similar to the generation of the resonance voltage. Accordingly, the control circuit having high resolution and capable of fine frequency control has been used in order to finely set the resonance frequency. However, there is also a problem that it is difficult to achieve fine adjustment of the high-frequency current by use of the control circuit incapable of performing fine frequency control.